Essentially, the observable universe is flat. Which means that the universe is far far larger, otherwise we would have seen some angle. Same as how you look, the Earth looks flat, but is is so larger than what you can see that eventually bends and you get a sphere. So the universe might be something like that too, though in that case it needs to be far larger than what we see.

with regards to the multiverse, there are several universal constants (such as the speed of light). Some of them are incredibly fine tuned, with one of them (the cosmological constant) being ridiculously fine tuned to support the creation of atoms, stars, galaxies and eventually life. Essentially leaving either a creator, or a large number of universes each with its own set of constants, and us being in one of the lucky ones which support life. The number of them is estimated to be above 10^500 based on the number of Calabi Yau spaces (string theory). Of course, just hypothetical at this stage.

There could be a number of infinite infinities. For example there is an infinite number of real numbers between 0 and 1. Same between 1 and 2. If you take each of these sets from 0 to infinity, you get an infinite of these sets, each having an infinite amount of numbers. That doesn’t count the negative numbers so there is another superset there. Complex numbers for another one. So there could be something like that in the universe too.